The present disclosure relates to digital printing systems having plural tandem print or printing engines of the type with seamed endless photoreceptor belts.
By way of background, in a typical electrophotographic printing machine a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charge thereon in the irradiated areas to record an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, bringing a developer material into contact therewith develops the latent image. Generally, the electrostatic latent image is developed with dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto. However, a liquid developer material may be used as well. The toner particles are attracted to the latent image, forming a visible powder image on the photoconductive surface. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to copy media. Thereafter, the toner image is heated to permanently fuse it to the copy media.
It is highly desirable to use a photoconductive member of this type in an electrophotographic printing machine to produce color prints. In order to produce a color print, the printing machine includes a plurality of stations. Each station has a charging device for charging the photoconductive surface, an exposing device for selectively illuminating the charged portions of the photoconductive surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the respective electrostatic latent image. The images are developed, at least partially in superimposed registration with one another, to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a sheet. The transferred multi-color image is then permanently fused to the sheet forming the color print.
Electrophotographic printing machines to date use a photoconductive member that is a seamed belt coated with a photoconductive material. Images are laid down on the belt such that an interdocument zone follows the image area, and since the seamed area of the belt results in an image quality defect, the seam area of the belt is kept within an interdocument area. Thus, the interdocument zones are limited to receiving latent process control patches that enable the electrophotographic process to be monitored and controlled.
In tandem printing systems, it is common practice to invert the sheet after print on one side thereof in a first of the print engines and for feeding the inverted sheet into a second print engine for print on the opposite side of the sheet to thus facilitate high speed duplex digital printing. However, in printing systems of this type arrangement, problems have been encountered in proper registration of the leading edge of the inverted sheet onto the photoreceptor of the second printing engine for proper placement of the image on the sheet and for avoiding the seam in the photoreceptor of the second print engine. Where the inverted sheet from the first print engine is transported by a transporter to the second print engine, errors in timing, transport speed and positioning of the sheet can accumulate to cause misregistration of the sheet on the second photoreceptor. This is particularly troublesome in view of the requirement that the sheet be placed on the second photoreceptor within a window of plus or minus 30 milliseconds timing with respect to the movement of the photoreceptor. Typically, tandem print engines employed for duplex printing operate to synchronize the position of the seams by varying the speed of the photoreceptor in the second print engine and can result in problems with front to back image-to-paper registration due to paper shrinkage from heating in the first print engine's fuser and differences in the photoreceptor belt length causing varied photoreceptor speed.
Digital printing systems employing tandem print engines for duplex printing have operated in accordance with a procedure wherein the system schedules the arrival times of the sheet stock in the initial and subsequent print engines and proceeds to have the feeder eject the sheet stock to meet the scheduled arrival time. The sheet then arrives at the entrance of the first print engine and is registered thereon for upper registration for print. The sheet is registered for image transfer from the photoreceptor belt and arrives at the discharge exit at the first print engine. The system then submits the sheet stock to the inverter, which discharges the sheet stock after a fixed dwell time.
Thus, it has been desired to provide a way of improving the registration of the leading edge of sheets emanating from a first tandem print engine onto the second print engine.
In a tandem print engine using seamed photoreceptor belts (or seamed intermediate transfer belts), there now exists a belt sync routine that adjusts the speed of the belt in print engine 2 so that the period of belt 2 is equal to the period of belt 1 in print engine 1. The seam of belt 2 is held in a constant phase offset (seam offset) with the seam of belt 1. This seam offset is chosen so that the media traveling through the media path will arrive at the transfer of print engine 1 and print engine 2 at the appropriate time—so that the sheet lead edge will meet the appropriate image panel on each belt. A new seam offset would need to be calculated for every sheet size, which requires another belt sync. Since the belt sync routine requires dead-cycling over multiple belt revolutions, this would have a significant productivity impact for customers running jobs with multiple sheet sizes.
Thus, the exemplary embodiments relate to a new and improved method and apparatus that resolves the above-referenced difficulties and others.